基于调频光谱原理，提出一种可用于在线测量的表面等离子体共振光纤传感器结构。理论分析、数值仿真和实测结果表明，基于表面等离子体共振效应的光纤传感头，相当于具有中心对称吸收谱的吸收介质，且吸收峰中心波长随待测折射率的增加而红移；基于此吸收谱函数，以微分光谱替代直接光谱测量,可有效克服吸收峰处一阶微分谱为零对测量灵敏度的消极影响；参考光路及闭环负反馈结构的引入，可有效抑制源于光源和光路中光强波动等的影响。以镀有50 nm厚金薄膜的多模石英光纤组成传感头，搭建相应的闭环系统，对折射率为1.33000～1.43000范围内不同液体进行折射率测量，并与阿贝折射率仪、光纤光谱仪及理论仿真结果进行比较。结果表明, 在1.33200~1.37580 RIU(RIU表示单位折射率)范围内，测量精度可达0.00016 RIU，与波长分辨率为0.4 nm的光谱仪测量结果精度相当；在1.37500~1.42640 RIU范围内，测量精度可达0.00071 RIU，优于0.00090 RIU的光谱仪测量结果，较好地验证了该方案用于在线测量的可行性和检测结果的可靠性。

An optimal structure of on-line optical fiber surface plasma resonance (SPR) sensors is proposed based on principle of frequency modulation. On the basis of theoretical analysis, numerical simulation and measured results, the optical fiber SPR sensing probes can be regarded as an absorbing medium with a centro symmetric absorption spectrum function, whose central wavelength at absorbing peak has a red-shift as increasing the refractive index of measured medium. The features of this centro symmetric absorption spectrum function make it possible to overcome the negative effect on measurement sensitivity originating from zero-value point of differential spectra at absorbing peak in directly measuring spectral method by the application of frequency modulation principle. And the adverse effects, rising from the intensity fluctuation of light source and in light propagating paths, can be suppressed by introduction of reference light path and negative feedback closed-loop system structure. The probes, which are made up of 15-mm long silica multi-mode optical fiber plated with 50-nm thick gold film, are applied in the proposed model, the corresponding experiments are arranged to measuring different liquid mediums with the index range of 1.33000~1.43000 refractive index unit (RIU).Then, discussion and analysis are carried out based on the data, which are got from the proposed method and other methods, such as Abbe refractometer, optical fiber spectrometer and numerical emulation. The results indicate that the precision of the proposed model is 0.00016 RIU at the range of 1.33200~1.37580 RIU, which is the same as measured results by applying optical fiber spectrometer with 0.4 nm wavelength resolution, and the precision is 0.000710 RIU at the range which is better than measured results by using optical fiber spectrometer with 0.0009 RIU. Therefore, the feasibility and reliability of the proposed method and the application for on-line measurement can be confirmed.